I already have too many irons in the fire, but I am getting started on plans for a foamie 1:10 scale Grumman Goose... that's almost 60" wingspan.

Lately, I've been building a lot of planes from a combination of 1/2" and 1/4" sheet foam... rigid insulation from the local hardware store. Simple slab-sided fuselages make for easy construction, and the wings will be typical spar and ribs wrapped with 1/4" FFF.

For extra strength, I cover the foam with newspaper and Polycrylic. This makes for a very lightweight yet stiff airframe... half the weight of a balsa/ply model so I'm not spending a fortune on motors and electronics. The WBPU also seals the foam so I can use cheap rattle-can paint without melting it.

I have completely redrawn my plans after I found a much better set of 3-views and sections... I should have the formers and ribs worked out today.

I plan to fly this plane almost exclusively from water, so I don't plan on making working retractable wheels... but maybe I'll provide for the option to manually install wheels for occasional grass operations. Besides, that's a lot of extra weight and complexity for this type of landing gear -- not worth it to me. However, I am giving serious thought to making working retractable wingtip floats... an upgrade performed on many late-model Goose's. It would be very easy to do with a standard set of 90-degree retracts... just need plenty of reinforcement (or make them break-away) so they don't rip out if I catch one.

A little more progress. I'll keep all the updated plans in post #1 for easy reference. In the cutaway drawings, the pink parts are from 1/2" rigid foam and the blue is 1/4" FFF. Using a wood spar in each wing, joined with a fiberglass arrow shaft.

I'm still working on the front view... am considering piecing it together with Google Sketchup to test-fit the parts.

And a couple more images added to post#1... formers and such. You can see that I am contemplating retractable wingtip floats... should be pretty easy to do. The wingtip floats are quite a bit more outboard than the fixed float positions... hope it doesn't affect water handling.

As far as I know, yes, Polycrylic and WBPU are the same. I also use Mod-Podge which is available at craft stores... decoupage glue that I believe is a mix of white glue and acrylic.... easy to apply when watered down slightly.

I use a soft paintbrush and brush it out as I lay it on to keep it smooth. Also, newspaper expands when it gets wet and shrinks as it dries... makes sure it is good and soaked before applying it, then brush/squeegee out the excess... as it dries it should tighten up.

For WBPU or Mod-Podge I get the paper completely soaked with glue before applying it. Messy, but it works well. When you first apply the paper you'll start worrying about the added weight.... it does feel heavy when it's still wet. But as all the moisture evaporates, it drops weight... might take a day for Mod-Podge or a few days for Polycrylic to fully dry out.

A different method of applying paper is to use 3M Super77 spray adhesive... simply spray the back of your paper then let it get tacky before putting it on the foam. If the adhesive is too wet then it will melt the foam... so you have to be careful. Works great for large flat surfaces like wings. Some even print graphics on the paper first, then glue it in place. Then you still have to seal the paper, which you can do in light coats with a spray lacquer.

Images

Brown paper is pretty heavy... that's why I use newspaper or even gift-tissue. Using any kind of glue, you have to be sure to squeegee out the excess to reduce weight. Mostly I like to cover with paper to withstand hangar rash... my planes get more damage just loading and unloading into the car than they do flying. The extra skin goes a long way to preventing little dings and creases. Plus, after covering with paper, you can use cheap rattle-can paint for color.